Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865 – 872
www.elsevier.com/locate/pnpbp
Review article
The hyper-systemizing, assortative mating theory of autism
Simon Baron-Cohen ⁎
Autism Research Centre, Department of Psychiatry, University of Cambridge, Douglas House, 18b Trumpington Road, Cambridge, CB2 2AH, UK
Accepted 10 January 2006
Available online 7 March 2006
Abstract
The hyper-systemizing theory of autism proposes that the systemizing mechanism is set too high in people with autism. As a result, they can
only cope with highly lawful systems, and cannot cope with systems of high variance or change (such as the social world of other minds). They
appear ‘change-resistant’. This proposal extends the extreme male brain theory of autism. Finally, evidence is reviewed for autism being the
genetic result of assortative mating of two high systemizers.
© 2006 Elsevier Inc. All rights reserved.
Keywords: Asperger Syndrome; Assortative mating; Autism; Systemizing
Contents
1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1. Systemizing non-agentive change . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Systemizing agentive change . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3. Systemizing in the general population (Levels 1–4) . . . . . . . . . . . . . . .
1.4. Systemizing in the autistic spectrum (Levels 5–8) . . . . . . . . . . . . . . . .
1.5. Autism as a result of assortative mating of two high systemizers . . . . . . . . .
1.6. Hyper-systemizing vs. weak central coherence vs. executive dysfunction theories
1.7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
There are two major ways to predict changing events. If the
event is agentive, one can adopt the “intentional stance” (or
‘empathize’). If the event is non-agentive, one can ‘systemize’.
In this article I outline a new theory that the systemizing mechanism has variable settings, and that people with autism
spectrum conditions are hyper-systemizers, who can therefore
Abbreviations: AS, Asperger Syndrome; AQ, Autism Spectrum Quotient;
EDD, Eye Direction Detector; ID, Intentionality Detector; PPQ, Physical
Prediction Test; SM, Systemizing Mechanism; SQ, Systemizing Quotient;
TOMM, Theory of Mind Mechanism; SAM, Shared Attention Mechanism;
TED, The Emotion Detector; TESS, The Empathy SyStem.
⁎ Tel.: +44 1223 746057; fax: +44 1223 746033.
E-mail address: [email protected].
0278-5846/$ - see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.pnpbp.2006.01.010
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only process highly systemizable (law-governed) information.
In line with the focus of this special issue of this Journal on
evolutionary perspectives, I also explore the evidence of the
assortative mating theory: that autism is the result of both parents
being high systemizers.
1.1. Systemizing non-agentive change
A universal feature in the environment that the brain has to
react to is change. There are at least two types of structured
change: (1) agentive change, (2) non-agentive change. Regarding the former, if change is perceived to be self-generated or selfpropelled (i.e., there is no apparent external cause), the brain
interprets it as agentive, that is, an agent with a goal. Goal
detection (or intentionality detection (ID)) is a fundamental
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S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
aspect of how the human brain interprets and predicts the behaviour of other animals (Baron-Cohen, 1994; Heider and Simmel, 1944; Perrett et al., 1985). Structured non-agentive change
is any change which is not self-propelled and where there is a
precipitating event (interpreted as a possible cause of the change)
or a pattern to the change. Some patterns are cyclical (the pattern
repeats every fixed number of units), but there are many other
types of pattern.
Structured non-agentive change occurs by degrees: some
change occurs with total (100%) regularity or pattern (e.g., the
sun always rises in the east and sets in the west). Other change
occurs with a lower frequency or regularity but there is still a
pattern to be discerned. Perception of structured non-agentive
change matters because the change might be injurious or have a
negative impact (e.g., planting your crops in February leads
them to wither) or a positive impact (e.g., planting them in March
leads to your crops thriving). Being able to anticipate change
thus allows the organism to avoid negative consequences or
benefit from positive change.
Systemizing is the most powerful way to predict change.
Systemizing involves law-detection via observation of inputoperation-output relationships (Baron-Cohen, 2002). Systemizing allows a search for structure (patterns, rules, regularities,
periodicity) in data. The goal of systemizing is to test if the
changing data is part of a system. Systems may be mechanical
(e.g., machines), natural (e.g., a leaf), abstract (e.g., mathematics), collectible (e.g., a collection), motoric (e.g., a tennis stroke),
or even social (e.g., the rules of etiquette). Thus, an engineer, a
lawyer, a mathematician, a film-editor, a librarian, an astronomer, a meteorologist, a chemist, a musician, a grammarian, a
company CEO, and a zoologist all systemize: they are all
concerned with formulating laws governing change (laws of
physics, laws of nature, mathematical laws, social laws, etc.,).
Systemizing allows the brain to predict that event x will occur
with probability p, that is, to identify laws driving the system.
Some systems are 100% lawful (e.g., an electrical light switch, or
a mathematical formula). During systemizing, the brain represents the information as input and output separately, so that the
pattern emerges (Tables 1 and 2). Systems that are 100% lawful
Table 1
Two examples of 100% lawful systems: (a) an electricity switch, and (b) a
mathematical rule
A.
Input = switch
position
Output = light
Up
Down
On
Off
Operation = switch change
B.
Input = Number
Output = Number
Operation = Add 2
2
3
4
4
5
6
Table 2
An example of systemizing hydrangea colouration
Hydrangea name
Acidic soil
Neutral soil
Alkaline soil
Annabelle
Ayesha
Alpengluhen
Altona
All Summer Beauty
Ami Pasquier
Amethyst
Bodensee
Blauer Prinz
Bouquet Rose
Breslenburg
Deutschland
Domotoi
Dooley
Enziandom
White
Blue
Purple
Blue
Blue
Purple
Blue
Blue
Blue
Blue
Blue
Purple
Blue
Blue
Blue
White
Purple
Red
Purple
Purple
Red
Purple
Purple
Purple
Purple
Purple
Red
Purple
Purple
Purple
White
Pink
Red
Red
Pink
Red
Pink
Pink
Purple
Pink
Pink
Red
Pink
Pink
Red
From http://www.hydrangeasplus.com.
have zero (or minimal) variance, and can therefore be predicted
and controlled 100%. A computer might be an example of a 90%
lawful system: the variance is wider, because the operating
system may work differently depending on which other software
is installed, or which version of the software you have, etc. The
weather may be a system with only moderate lawfulness.
A key feature of systemizing is that single observations are
recorded in a standardized manner. A meteorologist will make
measurements at fixed times and fixed places, measuring rainfall
(in a cup), temperature (with a thermometer), pressure (with a
barometer), wind speed (with an anemometer), etc. An astronomer will record the position of a planet at fixed times and fixed
places, tracking its movement. Such systematic data collection
(phase 1 of systemizing) can then lead to the observation of the
pattern of law (phase 2 of systemizing). Systemizing thus has the
power to reveal the structure or laws of nature.
1.2. Systemizing agentive change
Some aspects of agentive behaviour are highly lawful (e.g.,
cats typically use their right paw to swipe at a moving object).
Some human behaviour is also sufficiently scripted to be
moderately lawful (e.g., ballroom dancing). Human behaviour
in film is of course highly lawful, since each time the film is
replayed, the characters will do and say the same thing. But
outside of these special cases, if there are laws governing human
behaviour, they are complex and the variance is maximal. Maximal variance means that when change occurs, it could occur in a
virtually infinite number of ways. Thus, a person's hands, eyes,
mouth, posture, and facial expression might change in one of
hundreds if not thousands of possible combinations. Nor is there a
one-to-one mapping between facial expression and the underlying mental state that might be causing such changes in the face
(Baron-Cohen et al., 2004). Nor do situations predict the subtlety
of emotions, since in the same situation different people react
differently. Finally, humans as moving, changing objects also
require the agent they are interacting with to respond. They talk,
and their words appear as novel, unique combinations on each
occasion, unlike scripted behaviour. The right response to their
S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
Box 1
Mechanisms for empathizing
The empathizing system has 6 key components:
(a) ID (the Intentionality Detector) automatically interprets or represents an agent's self-propelled movement as a goal-directed movement, a sign of its
agency, or an entity with volition (Premack, 1990),
and is evident from at least 12 months of age (Gergely
et al., 1995).
(b) EDD (the Eye Direction Detector) automatically interprets eye-like stimuli as “looking at me” or “looking at
something else”. Cells in the superior temporal sulcus
that respond to averted or direct gaze have been
identified via single-cell recording (Perrett et al.,
1985). EDD is active in early infancy (Connellan et
al., 2001; Vecera and Johnson, 1995).
(c) SAM (the Shared Attention Mechanism) automatically represents if the self and another agent are
perceiving the same event, by building ‘triadic’
representations. SAM is active from 9 months of
age (Scaife and Bruner, 1975).
(d) ToMM (the Theory of Mind Mechanism) allows an
epistemic mental state to be represented (Leslie,
1987), enabling understanding of false belief (Wimmer and Perner, 1983), and the relationships between
mental states. ToMM is firmly established by 4 years
of age (Wellman, 1990).
(e) TED (The Emotion Detector) represents affective
states (Baron-Cohen, 2005). Infants can represent
affective states from as early as 3 months of age
(Walker, 1982). TED allows the detection of the basic
emotions (Ekman et al., 1972).
(f) TESS (The Empathizing SyStem) allows an empathic
reaction to another's emotional state (Baron-Cohen,
2005).
words isn’t to reply with a script. Agentive change in the social
world is too fast, and the laws — if they exist — are thus too
complex to systemize. Skinner (Skinner, 1976) claimed human
behaviour could be systemized if one had a complete record of all
the historical antecedents (A) and all the consequences (C) for
any piece of behaviour (B), such that A →B←C. The real social
world is, of course, not a Skinner Box.
Systemizing only works when one can measure or count one
thing at a time, ignoring or holding everything else constant.
Systemizing is enormously powerful as a way of predicting and
controlling events in the non-agentive world and has led to the
technological achievements of the modern world. It has this power
because non-agentive changes are simple changes to predict: the
systems are at least moderately lawful, with narrow variance.
Because ordinary social behaviour defies a systematic approach, the second-by-second changes in agentive behaviour are
more parsimoniously interpreted in terms of the agent's goals
867
(Baron-Cohen, 1994; Heider and Simmel, 1944; Perrett et al.,
1985). It appears that humans have specialized, inherited
‘hardware’ for dealing with the complex social world. The
‘empathizing system’ comprises basic instruments — like barometers, thermometers, and anemometers — that come compiled
to help the normal infant make sense of the social world, and
react to it, without having to learn it all from scratch. Empathizing is not the main focus of this article but is explained in
more detail elsewhere (Baron-Cohen, 1995, 2003, 2005; BaronCohen and Goodhart, 1994) (Box 1). Such basic modules or
neurocognitive mechanisms give the normal infant a foothold
into making sense of and responding to the social world. The
neural circuitry of empathizing has been extensively investigated (Baron-Cohen et al., 1999a,b; Frith and Frith, 1999; Happe et
al., 1996); key brain areas involved in empathizing include the
amygdala, the orbito and medial frontal cortex, and the superior
temporal sulcus. Experience allows us to learn the subtleties of
empathy, but such hardwired, innate mechanisms bootstrap the
brain to make rapid sense of social change.
The hyper-systemizing theory (Box 2) posits that we all have
a systemizing mechanism (SM), and this is set at different levels
in different individuals. The SM is like a volume control or a
dimmer switch. Genes and other biological factors (possibly
fetal testosterone) turn this mechanism up or down (Knickmeyer et al., 2005). For some people, their SM is set high so that
they systemize any changing input, analyzing it for possible
structure. A high systemizer searches all data for patterns and
regularities. For other people, their SM is set at a medium level,
where they systemize some but not all of the time. For yet other
people, their SM is set so low that they would hardly notice if
regularity or structure was in the input or not.
1.3. Systemizing in the general population (Levels 1–4)
Evidence suggests that within the general population, there are
4 degrees of systemizing: Level 1 corresponds to individuals who
have little or no interest or drive to systemize, and consequently
they can cope with total change. This would be expressed as a
talent at socializing alongside a lack of precision over details, and
as someone who can easily cope with change. Most people have
some interest in systems, and there are sex differences in this.
More females in the general population have the SM turned up to
Level 2, and more males have it turned up to Level 3. Those with
an SM at Level 2 might show typical female interests (e.g.,
emotions (Baron-Cohen and Wheelwright, 2003)), and those
with an SM at Level 3 typical male interests (e.g., in mechanics)
(Baron-Cohen, 2003). These differences might be quite subtle,
such that on a test of map-reading or mental rotation, males might
perform higher than females (Kimura, 1999). Some evidence
comes from the Systemizing Quotient, on which males score
higher than females (Baron-Cohen et al., 2003). Another piece of
evidence comes from the Physical Prediction Questionnaire
(PPQ), a method for selecting applicants for engineering. The
task involves predicting which direction levers will move when
an internal mechanism (of cog wheels and pulleys) of one type or
another is involved. Men score significantly higher on this test,
compared to women (Lawson et al., 2004).
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S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
Box 2
Summary of the hyper-systemizing theory of autism
Principal axioms in the theory:
1. The systemizing mechanism (SM) drives the brain to
look for input-operation-output relationships in any
data, and to construct systems.
2. In different individuals, the SM is set at different
levels, and the level of any given individual's SM is
determined by one's biology.
3. The higher the SM is set, the more one will attempt to
systemize, and the more one will be attracted by
systems with low variance; correspondingly, the
lower your SM is set, the more variance (or change)
one will tolerate in a system. The variance in a system
determines how lawful it is.
4. The higher one's SM is set, the more detailed one's
perception, because systemizing depends on details
as possible variables in the system.
5. In males, the SM is set at a slightly higher level than in
females.
6. People with Asperger Syndrome (AS) have their SM
set at a higher level than typical males.
7. Parents of children with autism spectrum conditions
have their SM set mid-way between people with AS
and typical males.
8. People with classic autism have their SM set at the
maximum level, and higher than people with AS.
9. Autism as a hyper-systemizing condition is postulated to be the result of assortative mating of two high
systemizers: in each parent, the SM is set at an above
average level, but the result in their child with autism
is that the SM is set to an extreme.
10. As a consequence, the child with autism is solely
attracted to systems of low or minimal variance, and
become distressed by systems (such as the social
world) where there is maximal variance.
Level 4 corresponds to individuals who systemize at a
higher level than average. There is some evidence that above
average systemizers have more autistic traits. Thus, scientists
(who by definition are good systemizers) score higher than nonscientists on the Autism Spectrum Quotient (AQ). Mathematicians (who by definition focus on abstract systems) score
highest of all the sciences on the AQ (Baron-Cohen et al.,
2001b). Another example of a group of people who are above
average systemizers is parents of children with autism spectrum
conditions (Baron-Cohen and Hammer, 1997; Happe et al.,
2001). The genetic implications of this are discussed later, as
these parents have been described as having the ‘broader
phenotype’ of autism (Bolton, 1996). At Level 4 one would
expect a person to be talented at understanding systems with
moderate variance (the stock market, running a company, the
law, engineering).
1.4. Systemizing in the autistic spectrum (Levels 5–8)
The autistic spectrum can be thought of as comprising at
least 4 subgroups: Asperger Syndrome (AS) (Asperger, 1944;
Frith, 1991), and high-, medium-, and low-functioning autism
(Kanner, 1943). They all share the phenotype of social
difficulties and obsessional interests (A.P.A., 1994). In AS,
the individual has normal or above average IQ and no language
delay. In the 3 autism sub-groups (high-, medium-, and lowfunctioning) there is invariably some degree of language delay,
and the level of functioning is indexed by overall IQ.1
Evidence suggests that people on the autistic spectrum have
their SM set at levels above those in the general population:
anywhere from Level 5 to Level 8. Level 5 can be seen as
corresponding to AS: the person can easily systemize totally
lawful systems (those that are 100% lawful, such as train timetables, historical chronologies) or highly lawful systems (e.g.,
computers) (Hermelin, 2002). They might also show an interest in
systems like the weather where the variance is quite high, so the
system is only moderately lawful (perhaps 60% lawful). The
clinical literature is replete with anecdotal examples (e.g., one
man with AS collected information of the type shown in Table 2
or Fig. 1), but there is also experimental evidence for superior
systemizing in AS: (i) People with AS score higher than average
on the Systemizing Quotient (SQ) (Baron-Cohen et al., 2003); (ii)
People with AS perform at a normal or high level on tests of
intuitive physics (Baron-Cohen et al., 2001a; Jolliffe and BaronCohen, 1997; Lawson et al., 2004; Shah and Frith, 1983); (iii)
People with AS can achieve extremely high levels in systemizing
domains such as mathematics, physics, or computer science
(Baron-Cohen et al., 1999a,b); (iv) People with AS have an ‘exact
mind’ when it comes to art (Myers et al., 2004) and show superior
attention to detail (O'Riordan et al., 2001; Plaisted et al., 1998b).
There is some evidence that in people with high-functioning
autism the SM is set at Level 6, in those with mediumfunctioning autism it is at Level 7, and in low-functioning autism
it is at the maximum setting (Level 8). Thus, the highfunctioning individuals who try to mentalize are thought to do
this by ‘hacking’ (i.e., systemizing) the solution (Happe, 1996),
and on the picture-sequencing task, they perform above average
on sequences that contain temporal or physical–causal (i.e.,
systemizable) information (Baron-Cohen et al., 1986). In the
medium-functioning individuals, in contrast to their difficulties
on the false belief task (an empathizing task) they perform
normally or even above average on two equivalent systemizing
tasks (the false photograph task (Leslie and Thaiss, 1992) and
the false drawings task (Charman and Baron-Cohen, 1992)). In
the low-functioning group, their obsessions cluster in the
domain of systems (Baron-Cohen and Wheelwright, 1999);
and given a set of coloured counters, they show their hypersystemizing as extreme ‘pattern imposition’ (Frith, 1970). Box 3
1
High functioning autism can be thought as within one standard deviation of
the population mean IQ (i.e. IQ of 85 or above); medium functioning autism
can be thought of as between one and three standard deviations below the
population mean (i.e. IQ of 55–84). Low functioning autism can be thought of
below this, (i.e. IQ of 54 or below).
S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
869
Fig. 1. An example of systemizing the weather, from the notebook of Kevin Phillips, a man with Asperger Syndrome. With kind permission.
lists 16 behaviours that would be expected if an individual had
their SM turned up to the maximum setting of Level 8.
The hyper-systemizing theory thus has the power to explain
not only what unites individuals across the autistic spectrum, but
why the particular constellation of symptoms is seen in this
syndrome. It also explains why some people with autism may
have more or less language, or a higher or lower IQ, or differing
degrees of mindblindness (Baron-Cohen, 1995). This is because,
according to the theory, turning the SM dial downwards from the
maximum level of 8, at each point on the dial the individual
should be able to tolerate an increasing amount of change or
variance in the system. Thus, if the SM is set at Level 7, the person
would be able to deal with systems that were less than 100%
lawful, but still highly (e.g., at least 90%) lawful. The child could
achieve a slightly higher IQ (since there is a little more possibility
for learning about systems that are less than 100% lawful), and the
child would have a little more ability to generalize than someone
with classic autism.2 The higher the level of the SM, the less
generalization, since systemizing involves identifying laws that
might only apply to the current system under observation.
Systemizing a ‘Thomas the Tank Engine’ video (a favourite for
many children with autism) may not lead to a rule about all such
videos, but just a rule that applies to this particular one with this
unique sequence of crackles and hisses.3
2
I am indebted to Nigel Goldenfeld for suggesting this connection between
hyper-systemizing and IQ.
3
The ‘reduced generalization’ theory of autism (Plaisted et al., 1998a) is thus
seen as a consequence of hyper-systemizing, rather than as an alternative
theory. Reduced generalization has been noted in autism for many decades
(Rimland, 1964) but is not discussed in any functional or evolutionary context.
In contrast, systemizing (an evolved function of the human brain) presumes that
one does not generalize from one system to another until one has enough
information that the rules of system A are identical to those of system B. Good
generalization may be a feature of average or poor systemizers, whilst ‘reduced’
generalization can be seen as a feature of hyper-systemizing.
At Level 7, one would expect some language delay, but this
might only be a moderate (since someone whose SM is set at
Level 7 can tolerate a little variance in the way language is
spoken and still see meaningful patterns). And the child’s
mindblindness would be less than total. If the SM is set at Level
6, such an individual would be able to deal with systems that
were slightly less (e.g., at least 80%) lawful. This would
therefore be expressed as only mild language delay, mild
obsessions, mild delay in theory of mind, and stilted social
behaviour, such as attempts at systemizing social behaviour
(e.g., asking for affirmation of the rule: “You mustn't shout in
church, must you?”) (Baron-Cohen, 1992).
1.5. Autism as a result of assortative mating of two high
systemizers
It is well established that autism arises for genetic reasons
(Bailey et al., 1995; Folstein and Rutter, 1988; Gillberg, 1991),
but the evidence for systemizing being part of the genetic
mechanism for autism includes the following: fathers and
grandfathers of children with autism are twice as likely to work
in the occupation of engineering (chosen as a clear example of a
systemizing occupation), compared to men in the general
population (Baron-Cohen et al., 1997). The implication is that
these fathers and grandfathers (both maternal and paternal) have
their SM set higher than average (Level 4). Consistent with this,
students in the natural sciences (engineering, mathematics,
physics, all of which require developed systemizing in relation
to mechanical or abstract systems) have a higher number of
relatives with autism than do students in the humanities (BaronCohen et al., 1998). If systemizing talent is genetic, such genes
appear to co-segregate with genes for autism.
The evidence that autism could be the genetic result of
having two systemizers as parents (assortative mating) includes
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S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
Box 3
Systemizing mechanism at Level 8: classic, low-functioning
autism
What does it mean for one's SM to be turned up to
Level 8? The person by definition systemizes everything.
Since in the social world the information is too complex to
be systemized, such individuals focus on systems which
are totally lawful (that is, with zero (or minimal) variance).
Key behaviours that follow from extreme systemizing
include:
• highly repetitive behaviour (e.g., producing a sequence
of actions, sounds, or set phrases, or bouncing on a
trampoline);
• self-stimulation (e.g., a sequence of repetitive bodyrocking, finger-flapping in a highly stereotyped manner,
spinning oneself round and round);
• repetitive events (e.g., spinning objects round and round,
watching the cycles of the washing machine; replaying the
same video 1000 times; spinning the wheels of a toy car);
• preoccupation with fixed patterns or structure: (e.g.,
lining things up in a strict sequence, electrical light
switches being in either an ON or OFF position throughout the house; running water from the taps/faucet);
• prolonged fascination with systemizable change (e.g.,
sand falling through one's fingers, light reflecting off a
glass surface, playing the same video over and over
again, preference for simple, predictable material such
as “Thomas the Tank Engine” movies);
• tantrums at change: as a means to return to predictable, systemizable input with minimal variance;
• need for sameness: the child attempts to impose lack
of change onto their world, to turn their world into a
totally controlled or predictable environment (a ‘Skinner Box’), to make it systemizable;
• social withdrawal: since the social world is
unsystemizable;
• narrow interests: in just one or two systems (types of
windows; catalogues of information).
• mindblindness: since the social world is largely
unsystemizable;
• immersion in detail: since a high systemizing mechanism needs to record each data-point: e.g., noticing
small changes;
• reduced ability to generalize: since high systemizing
means a reluctance to formulate a law until there has
been massive and sufficient data-collection. This could
also reduce IQ and breadth of knowledge;
• severe language delay: since other people's spoken language varies every time it is heard, so it is hard to systemize;
• islets of ability: since the high systemizer will channel
their attention into the minute detail of one lawful system
(the script of a video, or the video player itself, spelling of
words, prime numbers), going round and round in this
system to obtain evidence of its total lawfulness.
the following: (a) Both mothers and fathers of children with AS
have been found to be strong in systemizing on the Embedded
Figures Test (Baron-Cohen and Hammer, 1997). This study
suggests that both parents may be contributing their systemizing
genotypes. (b) Both mothers and fathers of children with autism
or AS have elevated rates of systemizing occupations among
their fathers (Baron-Cohen et al., 1997). (c) Mothers of children
with autism show hyper-masculinized patterns of brain activity
during a systemizing task (Baron-Cohen et al., in press). (d) The
probability of having a brain of Type S (Level 3) in the male
population is 0.44, and the probability of having a brain of Type
S in the female population is 0.14 (Goldenfeld et al., 2006). If
autism arises from assortative mating of two strong systemizers,
then the probability of autism in the population should be
(0.44 × 0.14) = 0.062. This is remarkably close to the actual rate
of autism spectrum conditions in the general population (Baird
et al., 2000; Fombonne, 2001). It is unlikely that the liability
genes for autism in males in the general population are common
polymorphisms, but that these are relatively rare in females in
the general population. Rather, it may be that in males the
liability genes interact with some other (endocrine?) factor to
increase risk, or that in females there is some protective factor
that decreases risk.
1.6. Hyper-systemizing vs. weak central coherence vs.
executive dysfunction theories
The hyper-systemizing theory predicts that when presented
with information or tasks that can be systemized, and especially
when presented with information that derives from a highly
lawful system, people with autism spectrum conditions will
perform at an intact or even superior level, always relative to a
mental age matched control group. Such an account differs from
the two dominant theories of the non-social features of autism,
the weak central coherence theory (Frith, 1989) and the executive dysfunction theory (Russell, 1997).
Regarding the former, it has been shown that people with
autism perform well on the Embedded Figures Test and on the
Block Design Subtest (Shah and Frith, 1983, 1993), and these
have been interpreted as signs of weak central coherence. But
given that both of these are lawful systems, the same data can be
taken as evidence for hyper-systemizing. People with autism
have been shown to have deficits in contextual processing
(Jolliffe and Baron-Cohen, 1999), but such material is harder to
systemize. Regarding the latter, people with autism show perseveration on the Wisconsin Card Sorting Test (Rumsey and
Hamberger, 1988), taken as a sign of an executive dysfunction.
But their perseveration on this task suggests that people with
autism spectrum conditions are focused on establishing a rule (a
key aspect of systemizing) and as good systemizers one would
not expect them to abandon testing the rule, but instead to keep
testing the rule, ignoring the experimenter’s request to shift to a
new, arbitrary rule. What appears as perseveration may therefore
be a sign of hyper-systemizing. Equally, people with autism may
take more moves on the Tower of London test (or its equivalents)
(Hughes et al., 1994), but if they are more focused on systemizing the task (identifying any lawful regularities), issues
S. Baron-Cohen / Progress in Neuro-Psychopharmacology & Biological Psychiatry 30 (2006) 865–872
such as solving the task in the minimum number of moves may
be irrelevant to them. We should be careful not to attribute a
deficit to people with autism spectrum conditions when they
may simply be approaching the task from a different standpoint
to the experimenter.
1.7. Conclusions
According to the hyper-systemizing theory, the core of autism
is both a social deficit (since the social world is the ultimately
unsystemizable domain) and what Kanner astutely observed and
aptly named “need for sameness” (Kanner, 1943). Autism is the
result of a normative systemizing mechanism — the adaptive
function of which is to serve as a law-detector and a changepredicting mechanism — being set too high. This theory
explains why people with autism prefer either no change, or
systems which change in highly lawful or predictable ways (i.e,
systems with ‘simple’ change, such as mathematics, physics,
repetition, objects that spin, routine, music, machines, collections); and why they become disabled when faced with systems
characterized by ‘complex’ change (such as social behaviour,
conversation, people’s emotions, or fiction). Because they
cannot systemize complex-change, they become “changeresistant” (Gomot et al., 2006).
Whilst autism spectrum conditions are disabling in the social
world, their strong systemizing can lead to talent in areas that
are systemizable. For many people with autism spectrum
conditions, their hyper-systemizing never moves beyond phase
1: massive collection of facts and observations (lists of dates
and the rainfall on each of these, lists of trains and their
departure times, lists of records and their release dates, watching
the spin-cycle of a washing machine), or massive repetition of
behaviour (spinning a plate or the wheels of a toy car). But for
those who go beyond phase 1 to identify a law or a pattern in the
data (phase 2 of systemizing), this can constitute original
insight. In this sense, it is likely that the genes for increased
systemizing have made remarkable contributions to human
history (Fitzgerald, 2000, 2002; James, 2003).
Acknowledgements
I am grateful for the support of the Medical Research
Council during this work. Matthew Belmonte, Marie Gomot,
Johnny Lawson, Jac Billington, Nigel Goldenfeld, and Sally
Wheelwright all contributed in important ways to the discussion
of these ideas. Matthew Belmonte also generously read the first
draft of this paper and provided valuable feedback.
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